Quadrupole rod set mass filters are well known and comprise four rod electrodes. FIG. 1 shows a typical arrangement of a quadrupole rod set mass filter. An analytical quadrupole 1 is preceded by a pre-filter quadrupole 2 and followed by a post-filter quadrupole 3. In operation, an RF voltage and a resolving DC voltage are simultaneously applied to the rod electrodes of the analytical quadrupole 1 so that the quadrupole rod set operates in a mass or mass to charge ratio resolving mode of operation.
As will be well understood by those skilled in the art, when the quadrupole rod set is operated in a mass resolving mode of operation, ions having mass to charge ratios within a desired mass to charge ratio range will be onwardly transmitted by the mass filter, but undesired ions having mass to charge ratio values outside of the mass to charge ratio range will be substantially attenuated.
Ions which are not desired to be onwardly transmitted by the mass filter are attenuated by causing the ions to assume unstable trajectories in the analytical quadrupole 1. As a result, at least some of the ions will impact upon the rod electrodes of the analytical quadrupole 1.
With increasingly bright ion sources, instrument robustness due to ion beam related contamination is becoming a serious problem. This is particularly true for quadrupole based instruments wherein ions across the majority of the mass to charge ratio range entering the mass filter will be lost to the analytical quadrupole rods during ion isolation. The ions lost to the analytical quadrupole rods build up and can eventually form non-conductive layers upon the rods forming the quadrupole rod set. The non-conductive layers which may form upon the rods can alter the fields within the quadrupole rod set thereby can effect analytical performance.
FIG. 2 of GB-2443952 (Micromass) discloses an arrangement wherein a mass spectrometer comprising an ion mobility spectrometer 8 and a mass filter 4 is provided. Ions emerging from the ion mobility spectrometer 8 are selectively transmitted or discarded using an ion gate 9.
U.S. Pat. No. 5,572,022 (Schwartz) discloses a mass spectrometer comprising a quadrupole filter.
WO 2004/06853 (Horning) discloses a mass spectrometer comprising a quadrupole ion accumulator.
It is desired to provide an improved apparatus for filtering ions and an improved method of filtering ions.